1. Field of the Invention
The present invention relates to a front conversion lens system included in a front converter which is mounted in front of an objective lens for converting a focal length of a photographing optical system from that of only the objective lens.
2. Description of the Prior Art
A camera is proposed, for example in U.S. Pat. No. 4,583,831, in which either a real focal length photographing mode and a pseudo focal length photographing mode is selectable prior to the photographing operation, and a mark, indicating the selected mode, is recorded onto a film with the photographed image at each frame. Such camera is called "trimming camera" hereinafter. When printing each frame of the film photographed by the trimming camera, the printer should discriminate the selected mode based on the mark recorded onto the film with respect to each frame to set its enlarging ratio in accordance with the selected mode.
In such a camera system including the trimming camera and the printer, an area to be reproduced on a print, if photographed in the pseudo focal length photographing mode, is smaller than a normal area of a frame photographed in the real focal length photographing mode. Therefore, if the pseudo focal length photographing mode is selected, such frame is enlarged with a larger ratio than that of the frame photographed in the real focal length photographing mode. Thus a print obtained by the pseudo focal length photographing mode is similar to a print photographed by using an objective lens having a longer focal length than that of the actual objective lens used in the photographing operation. Such apparent focal length is called "pseudo focal length" hereinafter.
Here, in the pseudo focal length photographing mode, the pseudo focal length is extended in accordance with the increase of the enlarging ratio in printing. Namely, the pseudo focal length is lengthened in accordance with the decrease of area in a frame to be reproduced on a print when the pseudo focal length photographing mode is selected.
However, in accordance with the increase of enlarging ratio, particles of a film become conspicuous in a print. Therefore, the pseudo focal length, that is, the enlarging ratio of the printer, is limited.
Here, the upper limit of the pseudo focal length should be limited according to a combination of, brightness condition of a main object to be photographed, exposure condition when photographed, characteristics of film and photographic paper to be used, and so on. However, it should be limited to 3 to 4 times the real focal length of the objective lens when considering the standard condition.